1. Field of the Invention
This invention relates to a process for producing a polycarbonate resin having good transparency and flowability, obtained by graft polymerization of an aromatic polycarbonate resin having at least one unsaturated terminal group with a vinyl monomer and provides a useful resin composition for optical uses such as optical discs, optical lenses and prisms.
2. Prior Art
So far known materials for optically transparent molded products include acrylic resin because of its distinguished characteristics such as good transparency and flowability and small birefringence [Japanese Patent Application Kokai(Laid-Open) No. 56-131654]. However, acrylic resin has a low heat resistance such as about 70.degree. C., a low impact resistance and a liability to warp due to moisture as disadvantages. To overcome these disadvantages it was proposed to use polycarbonate resin having a viscometric average molecular weight of 15,000 to 18,000 as molding materials for optical discs or lenses [Japanese Patent Application Kokai (Laid-open) No. 58-180553], but the proposed polycarbonate is not satisfactory in the flowability and still has a limit to its application.
To improve the flowability of polycarbonate resin, it was proposed to melt blend the polycarbonate resin with other thermoplastic resin such as polystyrene, polymethylmethacrylate, acrylonitrile-styrene copolymer (AS resin), maleic anhydride-styrene copolymer, etc., but it was difficult to make the sizes of dispersed particles less than 1 .mu.m due to the non-compatibility, resulting in optical un-uniformity. It was also proposed to use a vinylic monomer as a solvent for the polycarbonate resin and conduct bulk polymerization [Japanese Patent Application Kokai (Laid-open) No. 63-196612], where dispersed particles having sizes of less than 0.2 .mu.m could be obtained, but a larger amount of the solvent must be used, resulting in a costly disadvantage.
Moreover, Japanese Patent Application Kokai (Laid-open) Nos.54-117,555 and 53-31,796 disclose a process for producing a modified polycarbonate resin which comprises impregnating an aromatic polycarbonate resin with a vinyl monomer or a vinylidene monomer in an aqueous solvent to maintain an amount of free monomer to 50% by weight or below and then elevating the temperature of the aqueous dispersion liquid to polymerize the monomer. However, the thus obtained modified polycarbonate is insufficient for optical use.